Despite many efforts over the years, there has never been a fire-retardant asphalt roofing material having the same widespread acceptance as standard, non-fire-retardant versions.
The prior efforts have taken several directions: use of mineral fibers as a filler in the asphalt layers or as a replacement fiber in the roofing felt for the purposes of reducing combustible material and limiting flow and exposure of asphalt during a fire (see Fasold et al, U.S. Pat. No. 2,555,401; Tomlinson et al, U.S. Pat. No. 3,332,830; and Schuetz, U.S. Pat. No. 3,369,956); inclusion of chemical fire-retardant agents in the roofing (see Tomlinson and Bierly, U.S. Pat. No. 2,667,425); and/or use of extra or heavier layers of roofing granules. Some of these approaches have produced commercial roofing sufficiently fire-retardant to be rated Class A by Underwriter's Laboratory (in contrast to the Class C rating of standard asphalt roofing); but even those approaches are not the answer the art is seeking, since they either greatly increase the cost of roofing, require special manufacturing equipment or processes, or provide only marginal fire protection. As an example of the latter deficiency, some commercial roofing materials with glass fiber felts pass Underwriter's Laboratory's "burning brand" test on 1/2-inch-thick (2 centimeters-thick) roof decks, but they will not pass the test on 3/8-inch-thick (1-centimeter-thick) roof decks, which are now approved for use in construction.
A different approach tried by several prior workers is to introduce a layer of intumescable particles into the roofing, which, as stated in Donegan, U.S. Pat. No. 2,782,129, is intended to expand in the presence of a fire to form "a fire resistant support or rigid sponge which adsorbs the asphalt, preventing flow and providing an effective fire barrier to the underlying roof." Donegan suggests use of unexpanded vermiculite as the intumescable material, disposed as a particulate layer between two layers of asbestos-filled asphalt. Bick et al, U.S. Pat. No. 3,216,883, also suggests the use of vermiculite, either unexpanded or partially expanded, in "built-up" roofing (formed in place on a roof). Hinds, U.S. Pat. No. 3,365,322 (1968), cites disadvantages of vermiculite (it is expensive and, because of its low weight, is difficult to incorporate into roofing in uniform amounts), and suggests replacing the vermiculite with mineral granules that carry an intumescable coating of sodium silicate and borax.
None of these efforts with intumescable roofing have been as effective as some of the other described approaches. Roofing material as taught in Hinds was commerically sold for awhile, but without apparent success. Very little intumescence was provided by the coated granules, and fire-resistance appeared to depend on presence of asbestos fibers as a filler in the asphalt; such a filled asphalt is difficult to apply by standard coating equipment, is costly, and has toxicity and other disadvantages. In addition, the coated mineral granules were flood-coated into the roofing material at weights of 100 to 125 kilograms per 10-by-10-meter section of applied roofing, adding to cost and weight of the roofing. Also, the coating on the granules was soluble in water, and in the nearly continuous flood-coated layer was especially susceptible to leaching and consequent loss of intumescability.
Vermiculite as suggested by Donegan and Bick also offers only low-volume intumescence; and vermiculite will not intumesce until a fire has progressed sufficiently to create high temperatures.
In brief, nothing in the known prior work with intumescable roofing suggests that intumescence could be the basis for an effective and economical fire-retardant asphalt roofing.